The development demand is not a guarantee, and it can also come at the cost of destroying local businesses which cannot afford the downturn in customers during construction- many of which are small, minority owned sole proprietorships.

As for demand never changing... well, everything changes.

Source: Personal anecdata from living in an area where local government has been pushing more and more light rail development hard, in spite of never actually delivering any of the promised benefits to traffic congestion, ridership levels, community or business development.